1. Field of the Invention
This invention relates generally to a technique for repairing defects in a component made of a high temperature superalloy and, more particularly, to a technique for repairing cracks and/or high temperature oxidation/hot corrosion induced material losses in a vane for a turbine section in a gas turbine engine that includes depositing a superalloy powder or slurry in the cracks and then depositing a superalloy putty layer and a braze putty layer over the powder or slurry.
2. Discussion of the Related Art
The world's energy needs continue to rise which provides a demand for reliable, affordable, efficient and environmentally-compatible power generation. A gas turbine engine is one known machine that provides efficient power, and often has application for an electric generator in a power plant, or engines in an aircraft or a ship. A typically gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed air flow to the combustion section where the air is mixed with a fuel, such as natural gas, and ignited to create a hot working gas. The working gas expands through the turbine section where it is directed across rows of blades therein by associated vanes. As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby providing mechanical work.
Gas turbine engines of this type are periodically serviced for maintenance purposes. One of the maintenance operations is to detect erosion, mechanical fatigue and cracking in various turbine parts including high pressure and low pressure vanes in the turbine section of the engine. The hot working gas paths for the first and second rows of vanes in the turbine section is directly from the combustion section of the engine, which frequently causes erosion of the vanes at various locations and triggers thermal mechanical fatigue cracking. This causes the vanes to be reshaped, thus possibly directing the working gas in a non-optimal direction and could cause catastrophic failure. As the consequences of the erosion and cracking damages of the turbine vanes, engine operation efficiency is reduced and operation safety is jeopardized.
Because turbine vanes are made from either Ni or Co based vacuum investment cast grade superalloys, they are very expensive, and thus it is usually desirable to repair the above described erosion and cracking during turbine service instead of replacing the vane. Known processes in the art to repair erosion and cracks in turbine vanes include high temperature vacuum brazing or sintering and different types of fusion welding. Because fusion welding repair can cause base alloy cracking and component distortion issues, preferred repair technologies are vacuum furnace brazing/sintering technologies
Different furnace brazing/sintering technologies have been developed in the art to address defects of different types and sizes. For small size cracks, it is known in the art to employ transient liquid phase (TLP) bonding to repair the cracks. For large size cracks, three different approaches all employing wide gap brazing technologies have been employed in the art. Unlike TLP bonding, which uses an active diffusion braze alloy as a filler material, wide gap brazing or sintering is more complicated. One of the wide gap brazing techniques uses a single mixture of a braze/superalloy powder and organic binder formed as a slurry for large crack repair to reduce the total amount of Si, P or B eutectic phases at the braze layer location. Another wide gap sintering process is known, referred to as pre-sintered preforms (PSP), where braze/alloy mixtures are pre-sintered at certain temperature in a vacuum furnace to form a sheet type of preforms for subsequent component repair usage.
The third type of wide gap brazing technique is very different from the other two methods. Instead of mixing a braze with superalloy powders to form a single mixture filler, the method separately applies a braze material and a superalloy material to construct a two-layer filler structure. In the two-layer repair process, both the braze powder and the superalloy powder are applied in a putty form, where the putty is a mixture of the powder and an organic binder. The layers of the superalloy putty and the braze putty are then sintered in a vacuum furnace that causes the braze material to melt and infiltrate and consolidate the superalloy layer to be hardened. Hardening of the superalloy layer then will be realized through an active element diffusion process. As an extension of the double layer putty/putty method, double layer flexible braze tapes are also made using a similar approach in which more sophisticated organic binders are used for a long shelf life.
The TLP bonding process is typically suitable for repairing cracks with a gap size in the range of 0.05-0.25 mm and the single mixture wide gap repair process can be used to repair cracks with a gap size up to 1.5 mm. However, if the crack gap size is in the 0.3-1.5 mm range, the capillary effect of the crack weakens significantly, where inconsistent braze crack filling, formation of large shrinkage holes and the development of a centerline eutectic phase become difficult issues to overcome. In addition, as the gap size increases, a higher percentage of a brittle eutectic phase with a low re-melt temperature becomes a problem at the braze joint. As a result, TLP and the one mixture wide gap brazing approach are usually considered as cosmetic repairs. The pre-sintered preform method can only be used for a flat surface overlay repair as the PSP sheets are hard and brittle.
Wide gap crack repair techniques using the two-layer repair process have traditionally been preferred for damage sites that are widely open and easily to access. This is because both braze and superalloy powders are inserted into the damage sites in a putty form, where small and deep damage sites present challenges to get the putty within the damage cavities. Typically, it is necessary to open up the small defects through a mechanical grinding operation in order for the two-layer wide gap repair process to be performed where the defect size is increased. The grinding operation also removes component service induced oxides inside the cracks or on the surface of the erosion damage site so as to expose clean base metal for the brazing/sintering operation
The process of mechanical grinding converts small cracks into large cavities especially if the cracks are deep so that the superalloy putty filler material can be easily inserted into the opening formed by the grinding process However, the mechanical grinding process adds complexity and cost to the repair operation, and can significantly weakens the integrity of the original cast structure and increases the total amount of braze alloy required for the repairs. Increasing the total amount of braze alloy at the repair site can reduced the potential of any repeat repairs applicable to the same site. The outcomes are higher repair cost and lower repair quality. As hydrogen fluoride cleaning can effectively remove oxides from the damaged site with minimum negative impact to the base alloy, using mechanical blinding to serve solely for oxide removal is considered as an outdated method.